Missing Aragonitic Biota and the Diagenetic Evolution of Heterozoan Carbonates: A Case Study from the Oligo-Miocene of the Central Mediterranean

Abstract

Several previous studies have addressed the diagenetic evolution of heterozoan carbonate assemblages. Generally it is assumed that early diagenetic processes in heterozoan settings are mainly destructive, including abrasion and dissolution on the sea floor. Constructive diagenesis (cementation) is delayed to later stages in the burial environment, with pressure solution of calcitic grains acting as a cement source. This paper presents a study of Oligo-Miocene inner-to outer-ramp heterozoan carbonates from the Central Mediterranean (Maltese Islands and Sicily) indicating that early diagenetic processes are more important than previously assumed.Four to five different cement types, including fibrous, two types of epitaxial, bladed and blocky cement, are distinguished based on transmitted light microscopy. Cathodoluminescence microscopy allowed a differentiation between primary high-Mg calcitic (fibrous and epitaxial cement I) and primary low-Mg calcitic (epitaxial cement II, bladed and blocky) cements. Stableisotope data indicate cement precipitation from marine, marine-derived, and meteoric waters. Trace-element analyses point to cementation in an open system (Maltese Islands) and a closed system (Sicily).Our investigations show that the majority of constructive diagenetic processes in these rocks occurs rather early in the shallow, marine burial environment, which is transitional between the marine seafloor and the deep-burial diagenetic environment. The main cement source in this environment is assumed to be aragonite. We suggest careful consideration of the importance of aragonitic components in fossil heterozoan settings, which seem to be more abundant than previously assumed and can act as a major early cement source. Due to the low preservation potential of these components, detailed geochemical studies are necessary to detect aragonite as the cement source. Our findings also have implications when considering the reservoir qualities of these rocks, because primary porosity can be occluded early and secondary porosity is not preserved.

Decomposing Lithification Bias: Preservation of Local Diversity Structure in Recently Cemented Storm-Beach Carbonate Sands, San Salvador Island, Bahamas

“…In the latter case, cementation is usually linked closely with aragonite dissolution providing a local source of carbonate cement in various diagenetic environments (e.g., James and Choquette, 1990;Palmer and Wilson, 2004;Knoerich and Mutti, 2006;Tomašových and Schlögl, 2008). Stronger lithification will, therefore, be accompanied by an increase in the magnitude of diagenetic filters amplifying and potentially overwhelming sampling effects.…”

Echinoderms and Oligo‐Miocene Carbonate Systems: Potential Applications in Sedimentology and Environmental Reconstruction

“…These echinoids are especially important for diagenetic pathways including two generations of syntaxial cements. Echinoid content is equally high in samples from Ragusa Island (5-16 %, averaging around 9 %; see Knoerich & Mutti, 2006). …”

Heterozoan carbonates in subtropical to tropical settings in the present and past

“…An understanding of the carbonate depositional systems is also important for reservoir rock characterization. Architecture, ecological accommodation, facies distribution and diagenetic potential sensibly react to changes of the biological community (Pomar 2001a, b;Knörich and Mutti 2006).…”